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Polarization Engineering for Deep-Ultraviolet Light-Emitting Diodes

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Polarization Engineering for Deep-Ultraviolet Light-Emitting Diodes Rochester Institute of Technology RIT Scholar Works Theses 4-22-2020 Polarization Engineering for Deep-Ultraviolet Light-Emitting Diodes Cheng Liu [email protected] Follow this and additional works at: https://scholarworks.rit.edu/theses Recommended Citation Liu, Cheng, "Polarization Engineering for Deep-Ultraviolet Light-Emitting Diodes" (2020). Thesis. Rochester Institute of Technology. Accessed from This Dissertation is brought to you for free and open access by RIT Scholar Works. It has been accepted for inclusion in Theses by an authorized administrator of RIT Scholar Works. For more information, please contact [email protected]. R•I•T Polarization Engineering for Deep-Ultraviolet Light- Emitting Diodes by Cheng Liu A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Microsystems Engineering Microsystems Engineering Program Kate Gleason College of Engineering Rochester Institute of Technology Rochester, New York April 22nd, 2020 Polarization Engineering for Deep-Ultraviolet Light-Emitting Diodes by Cheng Liu Committee Approval: We, the undersigned committee members, certify that we have advised and/or supervised the candidate on the work described in this dissertation. We further certify that we have reviewed the dissertation manuscript and approve it in partial fulfillment of the requirements of the degree of Doctor of Philosophy in Microsystems Engineering. ______________________________________________________________________________ Dr. Jing Zhang Date Associate Professor ______________________________________________________________________________ Dr. Seth Hubbard Date Professor ______________________________________________________________________________ Dr. Stefan F. Preble Date Professor ______________________________________________________________________________ Dr. Parsian K. Mohseni Date Assistant Professor ______________________________________________________________________________ Dr. Stephen Boedo Date Professor Certified by: ______________________________________________________________________________ Dr. Bruce Smith Date Director, Microsystems Engineering Program ii ABSTRACT Kate Gleason College of Engineering Rochester Institute of Technology Degree: Doctor of Philosophy Program: Microsystems Engineering Authors Name: Cheng Liu Advisors Name: Jing Zhang Dissertation Title: Polarization Engineering for Deep-Ultraviolet Light-Emitting Diodes The new Coronavirus or Covid-19 has already became a world-wide pandemic. As of April 22nd, 2020, there are more than 2593129 confirmed case and 179725 total deaths. To fight against the Coronavirus, tremendous efforts have been made for slowing down the spread, such as social distancing. Recently, deep-ultraviolet (DUV) light-emitting diodes (LEDs) are demonstrated to be super effective to deactivate the virus/bacteria. In addition, the outstanding properties such as long life-time, compact size and environmental-friendly from DUV LEDs make them ideal for protecting people from the Coronavirus. However, all AlGaN-based DUV LEDs suffers from low external quantum efficiency issue. Among all the determinants, optical polarization significantly influences internal quantum efficiency and light extraction efficiency from the devices. In this work, we theoretically investigated the impact of the optical polarization properties on DUV LED device efficiencies by using a self-consistent 6-band k·p model and a finite- difference time-domain (FDTD) software. The factors influencing optical polarization switching are also examined and verified. In addition, this work proposed and demonstrated a novel delta-quantum well (QW) design to engineer the optical polarization from the active region. The molecular beam epitaxy (MBE)-grown AlGaN- delta-GaN structure in this study shows an extremely high 85% internal quantum efficiency at 255 nm on a conventional AlN/sapphire substrate, which indicates the delta- QW design is promising for efficient DUV LEDs. Furthermore, several other techniques, such as top-down nanowire arrays, sphere current spreading LEDs, AlGaN substrate and microsphere arrays, are also developed to further improve device efficiency. iii ACKNOWLEDGMENTS Pursuing this Ph.D. has been a truly life-changing experience for me, and I will never regret for this decision. There are many challenging moments during the past six years, and it would not have been possible to finish it without the support and help that I received from many people. First of all, I want to thank my parents, Jixiang Liu and Yumei Liu. Words cannot express how grateful I am for all the sacrifices that you’ve made. Also special thanks to my family, Sitong Zhou, Zhengzheng Zhou, Meiling Na, Zheng Liu, and Xu xu. Thank you for supporting and understanding me for everything. Love you all. I would like to express my appreciation to my advisor, Prof. Jing Zhang. You are a tremendous mentor who always encourages my research and guides me to grow as a research scientist. It’s my honor to be one of your first Ph.D. students, having the opportunity to build up the lab, to explore the research topics. The flexible and professional research environment you provide to us is of great importance for the success of my graduate study. I would also like to thank Prof. Seth Hubbard, Prof. Stefan F. Preble, Prof. Parsian K. Mohseni and Prof. Stephen Boedo for serving as my dissertation committees, and for sharing your brilliant comments and suggestions. Thanks to Microsystems Engineering Program at RIT, especially Prof. Bruce Smith and Lisa Zimmerman for all the help. Many thanks also to my lab mates, Yu Kee Ooi, Matthew Hartensveld, Bryan Melanson, Vijaygopal Thirupakuzi Vangipuram, and Gildas Ouin for letting my Ph.D. career to be an enjoyable experience. I will not be possible to get this work done without all your helps and contributions. I would also like to acknowledge our collaborators from Cornell University, Prof. Debdeep Jena, Prof. Grace (Huili) Xing, Kevin Lee, and SM Moududul Islam. Thank you so much to allow me to operate your Gen-10 Veeco MBE system, and for all the helps regarding the MBE growth and my projects. Thanks to all my friends for supporting me through all the difficulties, for bring me endless joy. I will always remember the burger nights at Blu Wolf Bistro drinking the “good” tequila with Michael Fanto and Zihao Wang, the mornings playing basketball and iv jogging around the campus with Xinye Chen, the days looking up at the galaxy and shooting arrows with Peichuan Yin, the weekends playing Tak and VR games with Brian Daly and Sarah Daly… I also want to thank my dog, the “drama queen”, Polymer, to bring me a lot of pleasure and “nightmare”. I also greatly appreciate the funding support from RIT microsystems Engineering Program, National Science Foundation, and Office of Naval Research. v CONTENTS LIST OF TABLES ix LIST OF FIGURES x CHAPTER 1: Introduction of III-Nitride Deep-Ultraviolet Light-Emitting Diodes 1 1.1 Applications of UV Light 1 1.2 Nitride Semiconductor for DUV LEDs 4 1.2.1 Fundamentals of III-Nitride Materials 5 1.2.2 Basics of LEDs 7 1.3 Current Status of DUV LEDs 11 1.3.1 Challenges for DUV LEDs 11 1.3.2 Radiative Recombination Efficiency 13 1.3.3 Light Extraction Efficiency 15 1.3.4 Current Status of DUV LEDs 16 1.4 Optical Polarization from DUV LEDs 17 1.5 Motivation of This Work 20 CHAPTER 2: Basic and Challenges of III-Nitride Deep-Ultraviolet Light-Emitting Diodes 29 2.1 Radiative Recombination Efficiency from Conventional AlGaN QWs 30 2.2 Carrier Injection Efficiency from Conventional AlGaN QW 34 2.2.1 Model of Current Injection Efficiency for AlGaN LEDs 34 2.2.2 Current Injection Efficiency from conventional AlGaN QW LEDs 36 2.3 Light Extraction Efficiency from Conventional AlGaN QW 39 2.4 Summary 41 CHAPTER 3: Optical Polarization from AlGaN-Based Deep-Ultraviolet Light- Emitting Diodes 43 3.1 Factors Influencing the Light Polarization Switching in AlGaN Layer 44 3.1.1 Strain State of the AlGaN Layers 44 3.1.2 Quantum Confinement in AlGaN QWs 45 3.1.3 Other Factors 47 vi 3.2 Effect of QW Thickness on the Optical Polarization of AlGaN-Based DUV LEDs 47 CHAPTER 4: Design Analysis of Al(In)GaN-Delta-GaN for Deep-Ultraviolet Emitters 62 4.1 Engineer of AlGaN-Delta-GaN QW for UV Emitters 63 4.1.1 Concept and Valence Band Structures of AlGaN-Delta-GaN QW 63 4.1.2 Optical Characteristics of AlGaN-Delta-GaN QW 66 4.1.3 Engineering of AlGaN-Delta-GaN QW 69 4.1.4 AlGaN-Delta-GaN QW for laser applications 71 4.2 Proposal and Physics of AlInN-Delta-GaN QW UV Lasers 72 4.2.1 Concept and Valence Band Structures of AlInN-Delta-GaN QW 73 4.2.2 Optical Properties of AlInN-Delta-GaN QW 76 4.3 Summary 79 CHAPTER 5: Polarization Properties of AlN-Delta-GaN Quantum Well Ultraviolet Light-Emitting Diodes 84 5.1 Physics and Optical Properties from AlN-Delta-GaN QW UV LEDs 85 5.1.1 Physics of AlN-delta-GaN QWs for deep- and mid-UV emissions 85 5.1.2 Optical Properties of MBE-grown AlN-delta-GaN QW UV LEDs 89 5.2 Polarization Properties from AlN-Delta-GaN QW UV LEDs 91 5.2.1 Optical polarization measurements setup 91 5.2.2 Optical polarization from AlN-delta-GaN QW UV LEDs 93 5.3 Summary 97 CHAPTER 6: MBE Growth of AlGaN-Delta-GaN Quantum Wells 101 6.1 Concept and Physics of 260-nm AlGaN-delta-GaN Quantum Well 103 6.2 MBE-grown 260-nm AlGaN-delta-GaN Quantum Well 104 6.3 Characterizations of MBE-grown 260-nm AlGaN-delta-GaN Quantum Well 106 6.3.1 Structural properties from the MBE-grown AlGaN-delta-GaN QW 106 6.3.2 High Internal
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